Northwestern began playing defense against Ohio State long before tonight’s game.

Northwestern, which has the smallest enrollment in the Big Ten at 19,000 students, has been working since the summer to keep Ohio State fans from overrunning Ryan Field when the teams kick off at 8 tonight.

“We owe it to our team to give them the best advantage possible, and if we can help set that up, we’ll do so,” said Ryan Chenault, assistant athletic director for marketing at Northwestern.

In years past, large numbers of Ohio State fans turned out to see the Buckeyes play at Ryan Field, which has a capacity of 47,130. And the problem hasn’t just been against OSU. Last season against Nebraska, Huskers fans were out in force and so loud that the Wildcats’ offense used a silent count late because its linemen couldn’t hear calls over the noise. Northwestern lost 29-28.

For tonight’s game, also a sellout, Northwestern implemented a new ticket plan — a type of dynamic-pricing model that acts as a descending auction — designed to cut out the secondary market by finding out how much people are willing to pay for a ticket.

The system, established in collaboration between the school’s economics department and its athletic department, meant that Northwestern began selling its top ticket for tonight’s game at $205. Others originally went for $151 (corner) and $126 (end zone). When those nearly 5,000 tickets went on sale on Aug. 1, Ohio State fans could have purchased Northwestern’s seven-game season-ticket package at these starting prices: $262 (sideline), $196 (corner) and $152 (end zone).

Northwestern also changed the way it made single-game tickets available to its fans with the hope that less would be sold to ticket brokers and then potentially resold to Ohio State fans.

The game is taking place on homecoming weekend, which has been drawing alumni to campus in increasing numbers recently, and Northwestern’s season-ticket base is at an all-time high and has more than doubled since 2009. (Northwestern wouldn’t reveal figures.) That should help offset the 3,450 tickets provided to Ohio State, which sold at $70 each.

Northwestern also changed its approach to single-game tickets. In July, the school offered about 100,000 alumni a chance to buy single-game tickets for the Ohio State game before they went on sale to the general public on Aug. 1.

Previously, season-ticket holders were permitted to buy an unlimited number of single-game tickets before they went on sale to the general public. Many of those tickets ended up being purchased by opposing fans from ticket brokers.

To combat that, Northwestern capped the number of additional single-game tickets that season-ticket holders could purchase. The limit was imposed only for “premium marquee” games against Ohio State and Michigan on Nov. 16.

Northwestern also implemented “Purple Pricing” for the marquee games, a system in which the school can lower ticket prices over time, based on demand, but never increase them. Fans could bid a price lower than the current price, which then placed the buyers in line to purchase the best-available seat if the actual price fell to their bid level.

When all tickets in a section sold out, everybody who bought one paid the lowest price. There is no reason to wait for a better price because Northwestern, through what it calls the “Purple Pledge,” refunds the difference. Northwestern priced tickets at the higher end to start, then lowered the price over time based on demand and what they are selling for on the secondary market.

“Those prices were triple the prices Northwestern had ever set for a football game, and they sold out weeks before,” said Northwestern economics professor Jeff Ely, one of the “Purple Pricing” co-creators. “We learned that for high-profile games — and this one for sure is the highest-profile game Northwestern has ever been in — there is a market.”

Tonight will show who in the market ended up with those tickets.

“In the past, we’ve had issues, and I know their fans will be here,” Northwestern coach Pat Fitzgerald said. “But our fans have been extremely supportive. Our ticket sales are at an all-time high. I’m expecting a very pro (Northwestern crowd) and home-field advantage for us.”